High-voltage PN diodes may generally be used for high-voltage applications. Advantages of high-voltage PN diodes are low reverse current and great robustness. The disadvantages are on the one hand the high forward voltage UF and on the other hand the high switching power dissipation.
In a high-voltage PN diode, voltage is accepted principally by the low-doped region, i.e. the space charge zone extends principally in the low-doped region. The doping concentration and thickness of this low-doped region are determined by the predefined breakdown voltage. A high breakdown voltage means a low doping concentration and a large thickness for this low-doped region.
When operating at high current density in the forward direction, high injection exists in high-voltage PN diodes, i.e. electrons and holes are injected into the low-doped region. With high injection, the concentration thereof is higher than the doping concentration of the low-doped region. The result is that the conductivity of the low-doped region is modulated, i.e. the conductivity becomes elevated. This reduces the forward voltage in advantageous fashion. The current of a high-voltage PH diode begins to flow at room temperature, however, only starting at approximately a forward voltage UF=0.7 V. Under normal operating conditions, e.g. at a current density>100 A/cm2, UF rises to values above 1 V. A correspondingly high, undesirable power dissipation is associated with this. Because a high-voltage PN diode requires a thick low-doped region, the voltage drop in the forward direction across the low-doped region is therefore relatively large despite conductivity modulation.
Upon shutdown, for example in the context of an abrupt current commutation, the charge carriers (electrons and holes) that are injected during operation in the forward direction into the low-doped region and stored there must first be dissipated before the high-voltage PN diode is at all capable of accepting reverse voltage again. In the event of an abrupt current commutation the current therefore at first continues to flow in the reverse direction until the stored charge carriers have been dissipated or cleared out. This current is also referred to as “reverse recovery current.” This operation, i.e. the magnitude and duration of the reverse recovery current, is determined chiefly by the volume of charge carriers stored in the low-doped region. The more charge carriers that are present, the higher the reverse recovery current. A high reverse recovery current also means a higher shutdown power dissipation. Integrating the shutdown current over time yields the reverse recovery charge Qrr, which is an important variable for describing the shutdown power dissipation and should be as low as possible.
In the design of high-voltage PN diodes, compromises must always be made between breakdown voltage, forward voltage, and shutdown power dissipation.
The shutdown power dissipation in high-voltage Schottky diodes is considerably lower as compared with high-voltage PN diodes.
The high-voltage Schottky diode is a so-called majority charge carrier component in which, even when there is a high current density during operation in the forward direction, no high injection occurs, i.e. no injection of electrons and holes into the low-doped region takes place during operation in the forward direction.
Because no high injection with conductivity modulation occurs with a high-voltage Schottky diode, however, a high voltage drops across the low-doped region when operated with high currents. This has hitherto limited the use of high-reverse Schottky diodes to very small currents. High-voltage Schottky diodes using silicon technology for high currents are therefore not known.